The present invention relates in general to vehicle electronic security systems, and, more specifically, to a method and apparatus for recovering an electronic transponder unit that experiences a programming failure during writing of a secret code into the unit.
Specially coded electronic transponders have been used as part of vehicle security systems to help ensure that access to the vehicle and/or starting of a vehicle engine is limited to a person carrying a transponder that is recognized by the vehicle. In one common form, a passive anti-theft system embeds a transponder in the head of a vehicle ignition key. When the key is turned in a lock in order to crank the vehicle engine, an electronic reader interrogates the transponder for a unique identification code that has been previously programmed into the reader. If the correct code is received, then the vehicle is allowed to start.
In order to avoid placing a power source such as a battery into the key head, a passive (i.e., batteryless) transponder capable of being charged electromagnetically by the reader has been employed. A charge pulse coupled from the reader to the transponder pumps up a charge on a capacitor that then supplies power to allow the transponder to transmit its identification code to the reader.
The earliest passive anti-theft systems transmitted information only in one direction (from the transponder to the reader). One potential vulnerability of such systems involves the cloning by an unauthorized person of the identification code into the transponder of another key unit. In this scenario, the unauthorized person obtains temporary possession of the legitimate key (e.g., at a valet parking service or during servicing of the vehicle at a repair shop) and interrogates it with a reader that then saves the identification code for later programming into another transponder. This facilitates stealing the vehicle at a later time.
To prevent such cloning of a transponder's code, systems with two-way communication have been introduced wherein the vehicle reader must authenticate to the electronic key before the electronic key will transmit the unique code that gains access to or starts the vehicle. The two-way authentication increases security and eliminates the ability of a potential thief to learn the secret transponder code without first knowing a unique, secret code given to the key by the vehicle reader during programming. Thus, a typical communication sequence during use of the security system involves 1) the electronic key providing an unprotected, freely-given ID code to the reader, 2) the reader using a secret encryption algorithm to encrypt the secret vehicle code and then sending it to the electronic key, 3) the electronic key decrypting the vehicle secret code and comparing it to its stored value for the vehicle secret code, 4) if the vehicle secret code matches, then the electronic key sending its secret password to the reader, and 5) the reader comparing the secret password with its stored values for all authorized keys and granting vehicle access accordingly. Typically, the vehicle secret code is unique to a particular vehicle and the vehicle uses only one secret code for all of its programmed keys. Alternatively, more than one secret vehicle code could be used.
The programming sequence used in connection with a two-way authentication system is necessarily more complex and requires a longer period of time than a one-way system. Besides transferring an identification code from the key transponder to the vehicle reader, a vehicle authentication code must be transferred to and stored in the key transponder. Various circumstances such as inadvertent removal of the electronic key from the reader before programming is completed, a power interruption during programming, or radio interference during programming can result in interruption of the process of writing a new secret code value. As a result of the interruption, the electronic key may hold an unknown value for the vehicle secret code and become unresponsive to further attempts to either program it or access it.